This invention relates to an optical information recording medium, such as a magneto-optical disk, which is used for recording information by an irradiation of a laser beam.
Various optical information recording media of the type are already known. For example, an optical information recording medium is disclosed by Sakuya Tamada and five others in Japanese Journal of Applied Physics, Vol. 28 (1989), Supplement 28-3, pp. 67-70, under the title of "Design Concept of Magneto-Optical Disk". Another optical information recording medium is disclosed by Goro Fujita and three others in Japanese Journal of Applied Physics, Vol. 28 (1989), Supplement 28-3, pp. 329-333, under the title of "A Magneto-Optical Recording Method of Magnetic Field Modulation with Pulsed Laser Irradiation".
In the manner which will later be described more in detail, a conventional optical information recording medium comprises a substrate and a single magneto-optical layer coupled to the substrate for recording the information. The substrate is made of polycarbonate and therefore is transparent for the laser beam. The magneto-optical layer is made of ferri-magnetic amorphous alloy which is composed of iron series transition metals and rare-earth transition metal.
For carrying out a recording operation of the information, various methods are already known. A particular one of the methods is disclosed in Japanese Patent Publication No. 60-48806 by Mimura et al. In the particular method, the laser beam is irradiated to the substrate with the magneto-optical layer subjected to a particular magnetic field representative of the information. In this event, the particular magnetic field has a polarity which is controlled to indicate the information. The laser beam is focused on the magneto-optical layer through the substrate. As a result, the information is recorded as recorded information in a particular portion of the magneto-optical layer in the manner known in the art. The particular method will be called hereunder a magnetic field modulation method.
With the magnetic field modulation method, it is possible to record different information in the particular portion of the magneto-optical layer without carrying out a deleting operation which is for deleting the recorded information from the particular portion. However, it is advantageous that a recording and reading characteristic is insufficient for practical use.